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Your instincts are correct. His installation is very dangerous.
12 gauge wire is not rated for 50 amps. It's not even rated for half that amperage. It's rated at 20 amps.
Since you have ordered #6 wire, you can only connect it to a 60 amp breaker or smaller (assuming it's copper wire).
You need 4 conductors between the two panels. If the enter distance between the panels will be piped in, you can easily add a ground wire, since you've ordered 6-3, especially in 2" conduit.
If the garage is detached from the house, you need a ground rod at the sub panel.
In the sub panel, ground wires and neutral wires are to be on separate bars, not mixed on the same bar(s).